Eliminate new warnings from clang 3.2
Add support for clang 3.2 and newer gcc compilers for Imageworks builds
Create option to use external (system) TBB.
imagespeed_test improvements:
1. --threads lets you test with different numbers of threads (default: 0,
   which means as many threads as cores available).
2. New iteration tests that include "slave" iterators, as we often use
   in ImageBufAlgo functions.
3. When more than one filename is provided, the read tests go through
   every file in sequence.  For the iteration tests, just the first file
   is used.
4. --autotile lets you set what size to test for autotiling.
5. --iteronly skips the lengthy image read tests, only times the iterators.
oiiotool --info now recognizes --metamatch and --no-metamatch
These control which metadata are or are not printed with oiiotool --info -v.

--metamatch is followed by a regular expression showing which metadata
to print (the rest will be excluded from output).
--no-metamatch is followed by a regular expression showing which metadata
NOT to suppress (the rest will be printed).
It's probably not very wise or intuitive to use both together.
Allow test output to exclude certain metadata when printing image info.

This uses the --no-metamatch option recently added to oiiotool.
This is really handy for certain tests that involve dumping oiiotool --info,
to simply make it not print the fields that might change from run to run
(like DateTime), or release to realase (like Software) and thus make
spurious testsuite failures.
Add testsuite for maketx
Fix SHA1.cpp build problem on some platforms -- issue #473
Fix testsuite reference output
Fix minor bugs that let IBA::fixNonFinite work properly with 'half' images.